Sectieoverzicht

    • There are some basic principles that you should follow when adding, or subtracting, measures of time. 

      Always:

      • add the 'like' units first 
      • start with the smallest unit
      • add seconds to seconds, minutes to minutes, and hours to hours
      • then simplify the answer when necessary by converting the units.

      Example: Adding time units

      Find the sum of 1 hour 55 minutes and 45 minutes.

      1 hr 55 min
              45 min
      ---------------
      1 hr 100 min

      You know that 60 minutes = 1 hour. Simplify 100 minutes by converting it in terms of hours and minutes:

      • 100 minutes = ____ hours ___ minutes
      • divide the number of minutes by 60
      • 100 ÷ 60 = 1 with a remainder of 40
      • 100 minutes = 1 hour 40 minutes.

      Rewrite 100 minutes as 1 hour 40 minutes. This will add 1 hour to the total number of hours:

      • 1 hr 100 min = 1 hr + 1 hr 40 min = 2 hr 40 min


      Example: Subtracting time units

      To subtract a measure of time, start by subtracting the smallest unit first. You may need to borrow from the larger unit. Then, simplify the answer when necessary by converting the units.

      Find the difference between 2 minutes 30 seconds and 1 minute 40 seconds.

         2 min 30 sec
      - 1 min 40 sec

      You cannot subtract 40 from 30. Borrow from the minute unit. 1 minute equals 60 seconds. Add 60 seconds to 30 seconds.

      The difference between 2 minutes 30 seconds and 1 minute 40 seconds is 50 seconds.

      Time can also be described as fractional units of time, for example: "a quarter of an hour" or "half an hour."  You know that 1 hour equals 60 minutes. To find a fraction of an hour, in this example a quarter of an hour, multiply the fraction by 60 minutes.

      \( 1/4 (60) = 60 \div4 = 15 \)


    • Elapsed time

      Elapsed time tells us how much time passes between two events. We can calculate the elapsed time by figuring out the difference between a start and a stop time.

      Example 



      School starts at 8am. The first break is at 10am. How many hours until the first break?



      This is an easy example to answer since both of the times are 'am' so you can just subtract the two, i.e. 10am - 8am = 2 hr.

      However, events can also occur before noon (am) and end after noon (pm). When that happens, finding the elapsed time requires an additional step, if you use the 12-hour clock:

      Example
      Hajierah goes to school at 8am and returns home at 3pm. How long was she at school? 

      You might, incorrectly think, that Hajierah has been at school for five (5) hours. But this is not the case. Before finding the difference, convert the times to the 24-hour clock:

      8am = 08h00
      3pm = 15h00 (3pm is 15 hours into the day, i.e. it is 15 hours after midnight)

      Now it is easy to work out how long Hajierah was at school. Simply subtract 08h00 from 15h00 and you will work out that she was at school for seven (7) hours.

      Now watch the video below which provides further examples on how to add, and subtract, time units. 

    • The Organic Chemistry Tutor. (2019). How to add and subtract time in hours and minutes (Standard YouTube licence)


      Test your understanding of adding and subtracting units of time, by answering the questions below.

    • Test your knowledge
      Leerlingen moeten
      Een cijfer behalen
      Het slaagcijfer behalen

    • Click on the activity link below to take part in the interactivity! Ensure that you select the option 'Guess' and then click on 'New Time'. You will be required to enter the elapsed time in hours. Click on 'Check' to see if you have answered correctly! The level of difficulty can be adjusted accordingly - have fun!


      Shodor Interactivate. (nd). Elapsed time (CC BY-NC)

    • Interactivity: Shodor Interactivate. (nd). Elapsed time (CC BY-NC) URL
      Leerlingen moeten
      Bekijken